A train leaves a town traveling at a speed of 16 mph.

A train leaves a town traveling at a speed of 16 mph. 30 minutes later another train, traveling at 64 mph leaves the same town in the same direction on a parallel track. How long will it take this train to catch up?

From the point of view of the second train, the first catches it up with the speed s=64-16=48 mph. When the second train starts, the first train is 16*0.5=8 miles away. Thus it will take the second train 8/48=1/6 hour or 10 minutes to catch the first train.